1. Field of the Invention
The present invention generally concerns PCM digital coding and decoding of speech signals and more particularly analog to digital error feedback coders which shape the spectral distribution of the quantizing error so as to reduce in-band noise.
More precisely, the invention concerns a digital transmission system, the transmit station of which includes a RC analog filter which limits the speech analog waveform to be transmitted to 4 kHz, a noise-shaping waveform coder of order higher than one, which codes the limited analog speech waveform into one-bit samples at an oversampling rate of 2.048 MHz, a digital filter which brings the sampling rate from 2.048 MHz to 8 kHz and the sample bit number from 1 to 12 and a compressor which brings back the sample bit number from 12 to 8. The receive station includes an expander which brings back the sample bit number from 8 to 12, a digital filter which oversamples the digital waveform from 8 kHz to 32 kHz, a Delta-Sigma digital decoder including a double integrator which converts the sampling rate from 32 kHz to 2.048 MHz and the sample bit number from 12 to 1, and a RC analog low pass filter.
2. Description of the Prior Art
The use of Delta-Sigma coders in PCM A/D converters has been proposed in the article "A single channel PCM coder" by J. D. Everard, IEEE, ICC 1978, Toronto, June 1978. In this article, the writer proposes to convert in the encode-direction the analog signal to digital form using a modified Delta-Sigma modulator operating at 2.048 kMHz, thus producing codewords with a single bit per sample magnitude. Conversion of the linear PCM codewords to compressed PCM is accomplished using a further digital transformation. In the decode-direction, the compressed PCM codewords are processed through a compressed PCM to linear PCM converter, then the linear PCM codewords are processed by a digital Delta-Sigma modulator to produce a single bit per sample code at 2.048 kMHz.
The sampling frequency of 2.048 kHz chosen by EVERARD does not allow to meet in Delta-Sigma coders the signal to noise ratio specifications recommended by the Comite Consultatif Telegraphique et Telephonique for PCM transmission systems (see CCITT, orange book, Vol. III-2, recommendation G 711, 712).
It has been also observed (see "Improvements to the Delta-Sigma modulators when used for PCM encoding" by J. D. EVERARD, Electronics Letters, July 22nd, 1976, Vo. 12, No. 15, pages 379-380) that the quantization noise distribution in Delta-Sigma modulators has an approximately square low relationship with frequency up to half the Delta-Sigma modulator sample rate. The exact distribution is intimately related to the amplitude probability density function of the input signal in such a way that very low output signal levels result in increased quantization noise within the signal band. This is undesirable for a PCM encoder application since very large clock rates are required to maintain adequate performance. Therefore, it has been proposed to increase the signal to noise ratio at low levels by injecting a jamming signal at such a frequency that it is filtered by the digital subsampling filter. This process is not satisfactory since it increases the signal to noise ratio at low levels but deteriorates it at high levels.
In brief, Delta-Sigma modulators used as A/D converters in PCM links cannot meet the CCITT specifications if the sample rate is not higher than 2.048 MHz and if the Delta-Sigma modulators are of the first order.
The object of the invention is to provide a Delta-Sigma coder of order two which operates at substantially 2 MHz and meets the specifications for A/D converters in PCM links.
Another object of the invention is to provide a Delta-Sigma coder which comprises a single operational amplifier and a single adder means.